linux-kernel - Re: [PATCH 2/2] ipc namespaces: implement support for posix msqueues

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Message-ID: <20081217190826.GA25890@us.ibm.com>
Date:	Wed, 17 Dec 2008 13:08:26 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.osdl.org>
Subject: Re: [PATCH 2/2] ipc namespaces: implement support for posix
	msqueues

Quoting Dave Hansen (dave@...ux.vnet.ibm.com):
> On Wed, 2008-12-17 at 11:55 -0600, Serge E. Hallyn wrote:
> > -void free_ipc_ns(struct kref *kref)
> > +void put_ipc_ns(struct ipc_namespace *ns)
> >  {
> > -       struct ipc_namespace *ns;
> > +       if (ns && atomic_dec_and_lock(&ns->count, &mq_lock)) {
> > +               mq_clear_sbinfo(ns);
> > +               spin_unlock(&mq_lock);
> > +               mq_put_mnt(ns);
> > +               free_ipc_ns(ns);
> > +       }
> > +}
> 
> OK, I see now why you wanted to remove the kref and add an atomic_t.
> That's probably worth a sentence in the patch description.

Good point, will do.

> I know that atomic_dec_and_lock() can be a nice logical thing to do, but
> is it necessary here?  Basically, is there something that happens under
> mq_lock that can re-raise the ns->count?

A racing task from another ipc namespace can open a mq file through
the VFS and thereby bump the ipc's refcount.  Will comment that here and
in the patch description.

thanks,
-serge
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